1. Field of the Invention
This invention relates to a method of compensating for atmospheric distortion while using a radar to detect the position of an object that is near the horizon relative to the radar site and, more specifically, to a method of determining the temporary bending angle of the atmosphere by analyzing the Doppler shift in signals from the apparent position of low-elevation man-made satellites to the known position of the satellite.
2. Background Information
Radar waves, like all light waves, are subject to distortion as the waves pass through the atmosphere and, more specifically, the troposphere. The distortion of the waves is caused by both large-scale effects and small-scale effects. Large-scale effects include refraction due to layers of gas with different densities in the atmosphere. Small-scale effects are caused by turbulence and heat gradients. Refraction by large-scale effects cause an object outside the atmosphere to appear in a location other than the object's actual location. Small-scale effects cause an object outside of the atmosphere to twinkle. The distortion due to large-scale refraction error for low elevation targets can be 2-10 mrad.
It is desirable to know the actual location of long range, low elevation objects such as, but not limited to, missiles as soon as possible so that an accurate trajectory may be determined. Thus, methods of correcting for the distortion caused by large scale effects have been developed. There are two common methods for determining the atmospheric “bending angle,” that is, the angle that the atmosphere bends light waves, caused by refraction. The Standard Atmospheric Model relies on a historical model of the atmosphere and on an assumption that earth and the atmosphere are spherical. This model is easy to compute and is functional for objects at higher elevations. However, the assumptions used to create this model result in a significant degree of error for objects at low elevations.
The second method relies on a three-dimensional model of the atmosphere based on National Weather Prediction data. The three-dimensional model is an improvement on the Standard Atmospheric Model as the assumptions are removed and replaced with data representative of the actual atmosphere. The National Weather Service collects data from satellites, balloons, ground stations and other sources and makes this data available to the public. The disadvantage to this method is that the National Weather Service data is only updated once about every six hours. Thus, the National Weather Prediction model relies on old data and may not be relevant in changing conditions.
There is a need for a method of compensating for atmospheric effects while using radar to detect an object near the horizon that determines a real time bending angle by comparing the Doppler shift in a signal from the observed location of a low elevation object to the signal from the apparent location of the low elevation object.
There is a further need for a method of compensating for atmospheric effects that reduces the refraction error for objects at a low elevation to less than 50 μrad.
There is a further need for a method of compensating for atmospheric effects that utilizes existing equipment.